After an upgrade on z-push, everything seemed to be syncing fine, but, under certain circumstances, sending of mail from an IOS device would fail. The log files hint that this is a WBXMLException and also notes that ZPush::CommandNeedsPlainInput(1) – this is a slight mis-direction. The solution is to ensure that the Virtual Host alias is set to the index.php page of your Z-Push web root.